The watchdog pclk is controlled from the secure GRF but we still
want to mention it explicitly to not use arbitary parent clocks
in the devicetree wdt node, so add a SGRF_GATE for it.
Suggested-by: Leonidas P. Papadakos <papadakospan@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>